Liquid impregnated capacitor

ABSTRACT

Electrical capacitors are impregnated with liquid dielectric compositions comprising mixtures of diaryl sulfones and esters of carboxylic acids.

1111 3,811,077 [451 May 14, 1974 Munch References Cited UNITED STATES PATENTS 11/1946 Clark............

[ LIQUID IMPREGNATED CAPACITOR [75] Inventor: Ralph H. Munch, Webster Groves,

Mo. v

[73] Assignee: Monsanto Company, St Louis, Mo.

. Primary Examiner-E. A. Goldberg [22] 1973 Attorney, Agent, or Firm-Wil1iam H. Dafi'ey [21] Appl. No.: 357,158

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8 Claims, 2 Drawing Figures PATENTEDHAY '14 974 FIG.2.

- boxylic acid.

, 1 LIQUIDIMPREGNATEUCAIMCITOR polymeric film; 'l"he diel'ectr icr sheet material? and? the interstices within the' dielectric she'etand between the dielectric sheet and. the conductors are. impregnated with a liquid dielec'tric composition: Suchrimpregnation is essential in ordertorealize the greatest dielectric. strength of theudielectric material.

The preferred dielectric. composition for impregnating capacitors has heretofore. been polychlbrinatedlbi phenyl which has a relatively high dielectric' constant and good low temperatureproperties;v Certain of the" polychlorinated biphenylshowever;havenow been dis covered to be resistant to natural degradation-s. and,

when released into the environment, these materials may enter the life cycle and be potentially harmfulito ecology. Evenathough' capacitors arerse'aledunits and; escape of the impregnant into the: environment? can -be prevented to alarge degree, ithasneverthelessibeeom'e' desirable to provide an alternate: composition. for 'im:--- pregnating' capacitors which doesnot contain: potenz tiall'y harmful polychlorinated biphenyl's.

It is accordingly an object ofi'thepresent inventionto provide a dielectric fluid composition: for impregnating; electrical capacitors which is free. of poly-chlorinated biphenyls. It is a furtherobject of this invention toapro-- vide capacitors-containing ,saidzalternate' dielectric. fluid composition. Other objectsof this invention will be-ap parent from. the:ensuingxdescription and claims;

SUMMARY The: compositions of the present invention: which are useful as impregnants for capacitors. of the type: having alternate layers of a metal foil conductor and a sheet of solid dielectric material comprise mixtu-resof adiaryl sulfone such as tolylxylyl' sulfone and an ester of a car- Capacitors containing said compositionsmay be constructed and impregnated accordingv to standard procedures. The dielectric sheet material interposed between the metal foil conductors may be paper, polymeric film such as polypropylene, or a combination of paper and film. Such capacitors impregnated-with the compositions of the presentsinvention arecharacterized by a low dissipation factor," highdielectric constant and good low I temperature performance.

DESCRIPTION OF DRAWING DESCRIPTION OF PREFERRED EMBODIMENTS The diaryl sulfone component of the compositions of this invention preferably comprises from about to 1 about 80 percent by weight of the total dielectric fluid wherein each Ar is individually a phenyl, napthyl, or

indane. radicaLeach-R is individually. a halogen or an alkyl;radicaliofoneitoabout eight carbon atoms, and

eachinisindiv-idually aninteger of from 0 to 3.

Representative diaryl sulfones emcompassed by the above-structure.are--diphenyl sulfone, ditolyl sulfone, diXylyl' sulfon-e, phenylxylyl sulfone, tolylXylyl sulfone, plienyltolylsulfone, indane phenyl sulfone, indane tolyl sulfone,,indane dinaphthyl sulfone, napthylphenyl sulfone; napthylxyly-l sulfone, and napthyltolyl sulfone.

Diaryl.sulfones ofthe present invention can be prepared, according to various. processes which. are well known: in: the art: See for example, U. S. Pat. Nos. 3,579,590, 3,125,604, 3,060,193, 3,057,952 and 3",04-5 ,050,.all-ofi whichaarei'ncorporated herein by reference.

Esters of carboxylic acids suitable for use in the present invention are: readily obtainable by acid-catalyzed esterification processes well known in the art. Such esterscan. be prepared by esterifying a carboxylic acid with: amal'cohol inthe presence of an esterificationcatal'yst',preferablyan'acid esterification catalyst, employing an. excess of alcohol to function as an entraining agent in. removing water formed in the reaction.

The carboxylic acid. reactant for preparing the ester canbeany organicxca-rboxylic acid..Non-limiting examplesof such acids are acetic acid, propinonic acid, butyric: acid, caproic acid, caprylic acid, capric acid, lauric. acid, tridecanoic acid, myristic acid, oleic acid,

' palmitic acid, stearic acid, acrylic acid, methacrylic acid, crotonicacid, isocrotonic acid, 3-butoanoic acid, sorbic acid,.malonic acid, succinic acid, adipic acid, pimeli'c acid, sebacicacid, dodecanoic acid, maleic acid, fiimaric acid, benzoic acid, napthoic acid, phthalic acid, isophthalic' acid, terephtalic acid, naphthalic acid, pyrornellitic acid, salicylic acid, azelaic acid, valeric acid,. hexanoicacid, heptanoic acid, octanoic acid, nonanoi'c..acid,.decanoic acid, glutaric acid and the toluicacids. It is understood that the corresponding anhydride may be used in place of the acid, e.g., phthalic anhydride.

Thus, typical carboxylic acids utilized in preparing the esters of this invention include aliphatic dicarboxylic acids having from 4' to 10 carbon atoms and aromatic dicarboxylic acids having from 8 to 16 carbon atoms.

Alcohols which may be used in the preparation of esters for purposes of the present invention include, but

are not limited. to, aliphatic alcohols such as methanol, ethanol, propanol, butanol, isobutanol, arnyl alcohol,

isoarnyl alcohol, hexyl alcohol, isohexyl alcohol, heptyl alcohol, octanol, isooctanol, Z-ethylhexanol, nonanol,

isononanol, vdecanol, isodecanol, undecanol, tridecano], phenylethanol, mixtures of C to C alcohols, and

. 3v cyclic alcohols such as cyclohexanol. Aryl hydroxides such as phenol and its derivatives may also be utilized.

- Thus, the ester component of the impregnantcompositions of this invention may be saturated orunsaturated m onoesters, diesters o r polyesters, and. may be aliphatic, aromatic or mixed aliphatic/aromatic ,in na- 'ture.

'alkyl esters may be'esters of mono-, dior polycarboxylic acids.

Preferred esters for purposes of this invention are dialkyl esters of dicarboxylic acids selected from the group consisting of adipic, axelaic, sebacic, phthalic, isophthalic, orthophthalic, terephthalic and mixtures of isomeric phthalic acids. Especially preferred herein are dialkyl esters of phthalic acid wherein the esterifiying alcohol is a mixture of C toC aliphatic alcohols, i.e., dialkylphthalate esters of C to C alcohols.

It is to be understood that where an ester disclosed and claimed herein is solid at room temperature, it is expedient and desirable to employ that ester in admixture with one or more liquid esters (and, of course, the sulfone) to facilitate impregnation of the capacitor.

' In addition to the diary] sulfone and the carboxylic acid ester, the dielectric fluid composition of this invention may contain minor amounts of numerous other components. In particular, it is often desirable to include a component to actas a stabilizer in the impregnated dielectric system. Generally the purpose of having a stabilizer in the system is to neutralize certain ionizable contaminants or extraneous materials which may be present or which may be-formed in the system. Such contaminants may include residual catalyst or catalyst activators which remain from resin forming reactions. Contaminants-may also include degradation products caused by environmental or voltage induced chemical reactions in the system. These undesirable contaminants and extraneous products have an adverse effect 'on' the dissipation factor or power facto'r of the impregnateddielectric system, and stabilizing agents have been found'to be highly effective in maintaining a low power factor in impregnated dielectric systems.

Examples of particularly preferred stabilizing agents methylcyclohexylmethyl-3 ,4-epoxy-6- methylcyclohexane carboxylate, and the like. These stabilizers are preferably employed in the dielectric fluid compositions of this invention in amounts in the general range of from 0.001 to about 8 percent by weight, and more preferably from about 0. l to 3.0 percent by weight. 1 v I 1 While the diary] sulfone component can comprise; from about 5 to about percent by weight of the dielectric fluid (capacitor impregnant) composition; of

this invention, it is advantageously employed in about 5 to about 50 percent by weight. Thus, the ester com ponent can be present in about 20 to about 95' percent by weight, preferably from about 50 to about 95 percent. These ranges can be adjusted, of course, to compensate for presence of the stabilizing agent.

One particularly preferred dielectric fluid composition of this invention comprises a mixture of about 10 percent by weight tolylxylyl sulfone, about percent by weight-of 'dialkylphthalateester wherein the alkyl groups are C, to C and about 0.3 percent by weight of 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate. r 1 r .The dielectric sheet materials interposed between the conductors in the capacitor and impregnated with the dielectric fluid compositions of this invention may be comprised of a solid flexible porous material such as highly refined cellulose paper, or of a substantially nonporous polymeric film material such as a polyolefin, or of a combination of paper and polymeric film. The paper material is preferably two or more sheets of Kraft capacitor paper having an individual sheet thickness not greater than about 1.0 mi]. and preferably about 0.3 mil. and a total combined thickness suitable for the design voltage of the capacitor. Such paper has a dielectric strength which is relatively good as compared to other dielectrics and has a relatively high dielectric constant. The polymeric material is preferably biaxially oriented polypropylene film although other members of the polyolefin family, particularly polyethylene and 4-methylpentene-l have found some use in capacitor applications. Other useful polymeric materials include polyesters, polycarbonates, polyvinylidene fluoride, andpolysulfone. Although either paper or polymeric film may be used alone, combinations of both are often .used. The paper is positioned adjacent the polymeric film to function as a wick to pass the dielectric liquid impregnant into the area coextensive with the area of contact between the porous paper and the substantially nonporous polymeric material.

Capacitor devices employing the present invention may have the general structure and configuration as shown in FIG. 1 which isa convolutely wound capacitor 10 comprising separate electrode foils or armatures 11 and 12 and intermediate dielectric spacers 13 and 14. Terminal connectors 15 and 16 gave enlarged surfaces (not shown) in contact with electrode foils l1 and 12. Electrode foils l1 and 12 may comprise one or more of a number of different materials, generally metallic and including for example aluminum, copper and stainless steel. Dielectric spacers 13 and 14 generally comprise paper and/or polymeric film as hereinbefore described. More specifically, the dielectric spacer 13 and the metallic electrode foils 11 and 12 taken together comprise a capacitor. element structure. The dielectric spacer materials, and the voids within and between the materials and the electrode foils are impregnated with a dielectric fluid composition.

Referring now to FIG. 2, there is shown an assembled capacitor unit 18 in which is encased a convolutely wound capacitor of the type shown in FIG. 1. The assembled unit includes a container 19, a hermetically sealed cover 20 which includes a small dielectric fluid fill hole 21 and a pair of terminals 22 and 23 projecting through cover 20 and insulated therefrom. Within the container 19 terminals 22 and 23 are connected to terminal connectors 15 and 16 shown in FIG. 1. Although not illustrated, the unit 18 shown in FIG. 2 further includes the dielectric fluid composition which occupies the remaining-space in container 19 not occupied by dielectric spacers l3 and 14.

Theimpregnation of the capacitor. a accomplished by using conventional procedures, For example, in one general impregnation method, capacitor units encased container 19 during the drying process.

The impregnating dielectric liquid is admitted to the capacitor assembly through hole2l preferably while the dried assembly is still under vacuum in a suitable evacuated enclosure. The capacitor element in the con tainer must be submerged by the impregnating liquid and usually enough of the impregnating liquid is introducedto completely flood the container and displace all the air therein. Thepressure of the enclosure is then raised to atmospheric pressure and the assembly permitted to stand or soak for a number of hours for thorough penetration of the liquid impregnant. After impregnation the capacitor unit may be sealed by applying a quantity of a suitable solder to hole 21 or by other suitable means. The capacitor assembly may thereafter be subjected to an elevated temperature to increase pressure within the capacitor assembly and aid the impregnation process. Heat and pressure may enhance impregnability by changing the relative wettability, viscosity and solubility of materials. in addition, expansion and contraction of individual components of the system which may be the result of heat and pressure may act as a driving force to induce migration of the liquid into the interstices of the dielectric spacer mate- .rial.

Numerous capacitors of the type illustrated in FIGS. 1 and 2 were constructed of aluminum foil and paper separators and were impregnated according to the foregoing description with a dielectric composition com-- prising 10.0 weight percent tolyixylyl sulfone, 89.7 weight percent diaikylphthalate (C to C and 0.3 weight percent 3,4-epoxycyclohexylmethyl-3,4- epoxycyclohexane carboxylate. Eight of these capacitors, designated Test Capacitors, were subjectedto an accelerated service test. The results of this test were compared to those obtained with'eight capacitors of identical type, the latter having been impregnated in a like manner with an electrical grade polychlorinated biphenyl containing about 42 percent chlorine, design'ated as Control Capacitors. The impregnant for the Control Capacitors contained 0.3 percent by weight 3,- 4-epoxycyclohexylmethyi-3,4-epoxyclohexane carboxylate.

Test conditions and results of the aforementioned testsare presented in Tables I and ii below. Table I pertains to the Test Capacitors and Table II pertains to the Control Capacitors.

iemperature, C 90 '90 90 90 Voltage 600 870 870, 870 870 Time, hours '0 130 307 475 715 CapacitorNo. 1: I

Dissipation factor .00345 .00305 .00302 .00208 .00294 Capacitance, microfarads- 1. 955 1.841 '1. 840 1. 841 1. 840 Capacitor No. 2:

Dissipation-factor .00335 .00314 .00306 .00303 .00304 Capacitance, microfara 1.958 1.341 1.841 1.841 1.841. Capacitor N0. 3:

Dissipation factor .00337 .00321 .00318 .00313 .00311 Capacitance, microi'arads 1. 930 1.817 1. 817 1.810 1.822

Capacitor No. 4:

Dissipation factor .00334 .00322 .00315 .00311 .00309 Capacitance, mitirofaradS--. 1. 961 1. 845 1. 844 1. S44 1. 844 Capacitor No. 5:

Dissipation factor .00334 .00327 .00322 .00319 .00321 Capacitance, microfarads 1. 918 1. 805 1. 807 1. 811 1. 815 Capacitor No. 6: I

Dissipation facto'r .00334 .00330 .00322 .00317 .00315 Capacitance, microfarads... 1.957 1.850 1.842 1.842 1.842 Capacitor No. 7:

Dissipation factor .00332 .00339 .00326 .00320 .00318 Capacitance, microfarads 1. 965 1.850 1.850 1. 850 1.850 Capacitor No. 8:

Dissipation factor .00833 .00331 .00328 .00323' .00319 Capacitance, microfarads. 1. 942 1.820 1.828 1.828 1. 828

TABLE I.T ES'I CAPACITOR" PERFORMANCE Tcst conditions:

Temperature 00 00 00 00 tagc. 000 870 870 870 870 Time, hours 0 130 307 475 715 Capacitor No. 1:

Dissipation facton... .00338' 00306 .00297 .00205 00293 Capacitance, microfarads... 1. 951 1. 836 1.835 1.836 1.835 Capacitor No. 2:

Dissipation factor .00337 .00311 .00303 .00302 .00299 Capacitance, microfarads. 1. 037 1. 822 1. 821 1. 821 1.820 Capacitor No. 3:

Dissipation factor .00334 .00325 .00313 .00312 .00311 Capacitance, micr01arads 1.954 1.836 1.835 1.835 1.835 Capacitor N o. 4:

Dissipation factor .00346 .00325 .00315 .00313 .00311 Capacitance, microfarads- 1. 966 1. 847 1. 846 1. 846 1. 846 Capacitor No. 5:

Dissipation factor .00347 00325 .00319 .00318' 00315 Capacitance, microfarads 1. 954 1.838 1. 837 1. 837 1.837 Capacitor N o. 6: Y

Dissipation factor 00332 .00334 .00321 .00310 00315 Capacitance, microfara 1. 062 1. 847 1. 846 1.846 1. 846 Capacitor No. 7:

Dissipation factor .00331 .00330 .00324 .00322 .00321 Capacitance, microfarads... 1. 953' 1.835 1. 834 1.834 1.834 Capacitor No. 8:

Dissipation factor .00344 .00332 .00324 .00322 .00311 Capacitance, microfarads 1. 954 1.826 1. 826 1. 826

" Two sheets of 0.66 mil. Kraft paper.

TABLE IL-CONTROL CAPACITOR" PERFORMANCE Test conditions:

* Two sheets of 0.66 mil. Kraft paper.

All 16 capacitors were subjected to test conditions which are considered extreme in comparison to normal commercial tests for'capacito rs. The test temperature was C. and the test voltage was 870 volts. Thus, the

test conditions of Tables I and ii exceed normal test temperatures by 20C. The rated capacitor voltage is herein.

fluid .for polychlorinated biphenyls, while retaining equivalent electrical performance, has

The preceding examples and data of Table I serve to illustrate a preferred embodiment of the invention, but the invention is not to be limited to the composition or capacitors defined therein. The invention encompasses generically defined dielectric fluid compositions as hereinbefore described, and capacitors containing such compositions. In addition, although the examples and discussions have been directed to capacitors constructed. of individual sheets of conductors and insulators, it is understood that metallized film canalso be used in conjunction with the dielectric fluid compositions defined herein and capacitors constructed therefrom are included within the scope of this invention. Accordingly, the invention is not to be limited except as defined by the claims appended hereto.

The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows:

l. An electrical capacitor comprising at least two electrodes and a dielectric sheet positioned therebetween, said dielectric sheet and interstices between said dielectric sheet and said electrodes being impregnated with a dielectric liquid composition comprising a mixture of a diaryi sulfone and an ester of a carboxylic acid. v

2. A capacitor of claim 1 wherein said dielectric composition comprises from about 5 to about 80 percent by weight of a diarylsulfone represented by thestrpcture been achieved I 4. A capacitor of claim 2 wherein the diaryl sulfone is present in an amount of from about 5 to about 50 percent by weight with thebalance of the dielectric composition comprising a.dialk yl ester of a dicarboxylic acid selected from the group consisting of adipic,

axelaic, sebacic, succinic, isophthalic, orthophthalic,

terephthalic and mixtures of isomeric phthalic acids.

5. A capacitor of claim 4 wherein the diary] sulfone is tolylxylyl sulfone. I

6. A capacitor of claim 5 wherein the ester is a dialkylphthalate ester of C to C alcohols.

7. A capacitor of claim. 6 containing from about 0.1 to 3 percent by weight 3,4-epoxycyclohexyl-m'ethyl- 3,4 epoxycyclohexane carboxylate.

8. A capacitor of claim 7 wherein the dielectric sheet is selected from the group consisting of paper, polyolefin film, and combinations thereof. :k

W050 UNITED STATES PATENT OFFICE CERTIFICATE OF. CORRECTION Patent NO. ZQQLLLLOYY Datd y 1; 197 Inventor) Ralph H. Munch It is certified that error appear. in the above-identified patent and that laid Letter: Paton: it. hereby corrected an shown below:

r y v a On Title Page, Attorney "William H. Daffey" should be William H. Duffey Claim 2, line 7 "1 to about 6" should be 1 to about 8 Claim 3, last vword "toms" should be atoms Claim l line l,= "claim 2" should be clai 3 001.6, line 30, Table II, "1,3 41" should be 1,8"1

Signed and sealed this 17th day of September 1974.

(SEAL) Attest:

C. MARSHALL DANN Commissioner of Patents MCCOY M. GIBSON JR. Attesting Officer P0405) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,8 l 1 077 Dated May 1", 197

humor) I Ralph H. Munch I: is certified that error appears in the above-identified patent and that and Letters Plant in hereby corrected as shown below:

F C On Title Page, Attorney "William H. Daffey" should be William H. Duffey Claim 2, line I, "l to about 6 should be l to about 8 Claim 3, last word "coms' should be atoms Claim 4, line 1, "claim 2" should be cl i 3 001.6, line 30, Table II, "1,3h1" should be 1,8u1

Signed and sealed this 17th day of September 1974.

(SEAL) Attest:

C. MARSHALL DANN Commissioner of Patents MCCOY M. GIBSON JR. Attesting Officer 

2. A capacitor of claim 1 wherein said dielectric composition comprises from about 5 to about 80 percent by weight of a diaryl sulfone represented by the structure
 3. A capacitor of claim 1 wherein the ester is selected from the group consisting of aliphatic dicarboxylic acids having from four to 10 carbon atoms and aromatic dicarboxylic acids having from eight to 16 carbon toms.
 4. A capacitor of claim 2 wherein the diaryl sulfone is present in an amount of from about 5 to about 50 percent by weight with the balance of the dielectric composition comprising a dialkyl ester of a dicarboxylic acid selected from the group consisting of adipic, axelaic, sebacic, succinic, isophthalic, orthophthalic, terephthalic and mixtures of isomeric phthalic acids.
 5. A capacitor of claim 4 wherein the diaryl sulfone is tolylxylyl sulfone.
 6. A capacitor of claim 5 wherein the ester is a dialkylphthalate ester of C7 to C11 alcohols.
 7. A capacitor of claim 6 containing from about 0.1 to 3 percent by weight 3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexane carboxylate.
 8. A capacitor of claim 7 wherein the dielectric sheet is selected from the group consisting of paper, polyolefin film, and combinations thereof. 